The present invention concerns a method in a cylinder dryer of a paper machine, in which a paper web is guided in conjunction with upper cylinders by utilizing an upper wire and in conjunction with lower cylinders by utilizing a lower wire. These wires are guided by the faces of the respective drying cylinders and by guide rolls situated in the spaces between the respective cylinders so that on an upper line of cylinders the web is pressed by the upper wire into direct drying contact with the faces of the upper cylinders and correspondingly the web is pressed by the lower wire onto the faces of the lower cylinders. Additionally, the web is passed from one line of cylinders to the other over a certain distance as an open draw.
The present invention additionally concerns a device for such a cylinder dryer of a paper machine in which a twin-fabric draw is utilized, and in which the device can be located in the spaces between the adjoining drying cylinders. Such spaces are defined by the drying wires running over the adjoining drying cylinders and by free sectors of the guide rolls guiding the drying wires. The device comprises a blow box which extends substantially over the entire width of the drying wires and in which there is at least one nozzle slot or a corresponding series of nozzle holes.
In the present application, a "single-wire draw" means such a mode of passing the web over the heated drying cylinders in which the web runs from one line of cylinders to the other supported by the drying wire, so that on one line of cylinders the web is situated between the drying wire and the cylinder face, while on the other line of cylinders the web is situated outside and the drying wire is situated between the cylinder face and the drying fabric or web, with the web running in the draws between the lines of cylinders while being supported by the drying wire. An advantage of this single-fabric draw is that the web is at all times supported by the drying wire, and has no open draws or at least no substantially long open draws, whereby risk of wrinkles and breaks in the web is reduced.
In the present application, a "twin-wire draw" means the prior art mode of supporting and drawing the web in conjunction with the heated drying cylinders, in which an upper wire is used in connection with the upper cylinders and a lower wire is used in connection with the lower cylinders. These wires are guided by the faces of the respective drying cylinders and by guide rolls situated in the spaces between the respective drying cylinders so that along the upper line of cylinders the web is pressed by the upper wire into direct drying contact with the faces of the upper cylinders, and the web is correspondingly pressed by the lower wire against the faces of the lower cylinders.
The present invention is expressly related to a twin-wire draw.
With a twin-wire draw, the web had usually has substantially long open draws as the web passes from one line of cylinders to the other. These open draws have been susceptible to fluttering and to resulting breaks and wrinkles in the web. This drawback has been manifested with particular emphasis in the initial part of the drying section, where the web is still relatively moist and therefore of low strength, and where its elastic properties are conducive to fluttering.
Present day opinions are that the fluttering of the web results mainly from excessively strong internal flows in a pocket and from differences in pressure prevailing both in pockets and also in nips formed by the web and the wire and by the cylinder faces. Such strong air flows and the differences in pressure that are produced are the results of boundary-layer flows, which are induced by the wires, by the web, and by the cylinder faces as they move.
In the prior art, attempts have been made to eliminate the drawbacks resulting from fluttering of the web, by making the open draws of the web in the initial part of the drying section shorter by placing the imaginary planes extending through the axes of the upper and lower lines of cylinders at a shorter distance from one another than what had been customary or what would be optimal, e.g., in view of the efficiency of drying.
Attempts have also been made to reduce the fluttering of the paper web in a drying group provided with a twin-wire draw by displacing the felt guide rolls so that the paper web has to run a shorter distance unsupported.
The open draws of the web, the free faces of the cylinders, as well as the wires or felts guided by guide rolls define pockets inside the multi-cylinder dryer. These pockets are open at the ends thereof but in other respects are closed. Ventilation of these pockets has been considered an important factor in view of the efficiency and uniformity of the drying (moisture profile) in a multi-cylinder dryer.
In a manner known in the prior art, the regulation of the moisture profile in these pockets has been arranged so that the blow pipes present in the pockets have been divided into blocks in a transverse direction of the machine. These blocks can be opened and closed so as to regulate the quantity of air blown into the pocket. With respect to this solution, reference is made to the Valmet FI Pat. No. 68,278.
In recent years, the running speeds of paper machines have been constantly increasing. Now, the limit of 1,500 m/min is already approaching. The fluttering of the open draws of the web is becoming an ever more serious problem, hampering the running quality of the paper machine.
A particular object of the present invention is to reduce the problems of fluttering described above.
With a twin-wire draw, a moist boundary layer is produced on the wire when the wire runs on the cylinder as water vapor evaporated from the paper is diffused through the wire. When the wire departs from the cylinder, this moist boundary layer goes through the wire into the pocket because of the negative pressure present in the nip.
With respect to the prior art most closely related to the present invention, reference is made to the Valmet FI Published Patent Application No. 73,259 and FI Patent Applications Nos. 854494 and 872691. The starting point of the present invention involves the methods and devices suggested in the above patents and documents. However, it has not been possible to break the above moist boundary layer produced on the wire in a twin-wire draw or to eliminate the drawbacks caused by this phenomenon, by means of these methods and devices.